In the manufacture of semiconductor products, substrates (e.g., semiconductor wafers) are processed by successively depositing, etching, and polishing various layers to create semiconductor devices. Within the semiconductor industry, it is common practice to utilize the benefits of a RF (Radio Frequency) driven plasma to dry etch material from a substrate. During plasma etch processes, it is paramount that the stability and uniformity of the plasma is controlled in order to improve process efficiency and yield for the substrate under process. This can be accomplished through a variety of methods, one of which is to control the plasma formation through the use of mechanical and electrical elements within the plasma chamber to confine the plasma to the process region of interest. These arrangements are well-known in the art and will not be elaborated further here.
While plasma confinement is a well understood practice; there is a need to detect when confined plasma changes to an unconfined state within a process chamber. Unconfined plasma can cause particle contamination and plasma non-uniformities, leading to degradation in the yield of the substrate under process and/or damage to the plasma processing system. A current scheme for detecting a change in plasma confinement employs a Si Carbide pin that is attached to the outer ring of the electro-static chuck (ESC) and in direct contact with the plasma. U.S. Pat. No. 5,737,177, issued Apr. 7, 1998, discloses one such scheme. While this instrument provides a relatively reliable means of detecting a change in plasma confinement by providing a DC (direct current) measurement of plasma sheath potential, it is a consumable part that must be replaced frequently and can induce plasma non-uniformities at higher RF powers and can result in particle contamination issues. Another scheme for detecting a change in plasma confinement is through the use of an OES technique (Optical Emission Spectroscopy) technique. However, it has been found that monitoring OES signals is an unduly complex and oftentimes unreliable method of detecting plasma unconfinement events.
Therefore, alternate methods and arrangements for detecting unconfined plasma events in a plasma processing system are desired.